Mr. Obama used the opening day of a Group of 20 meeting in St. Petersburg to press his Syria agenda, speaking for an hour with Japan's prime minister. With Mr. Putin, the summit's host, he simply shook hands and smiled for cameras, with no further meeting plans scheduled.

ENLARGE

The Minsk, a Russian Navy landing ship, passed Thursday through the Bosphorus Strait toward the Mediterranean as Putin welcomed Obama.
European Pressphoto Agency

Their tense standoff, in many ways, is the outgrowth of previously undisclosed calculations about the level of U.S. interest in the civil war in Syria. In early 2012, White House and State Department officials asked themselves what the U.S. might be willing to do to wean Russia from its support of Syrian President Bashar al-Assad. Curtail missile defenses in Europe? Pare plans to enlarge the North Atlantic Treaty Organization?

Russian President Vladimir Putin welcomes President Barack Obama at the beginning of the G-20 summit in St. Petersburg, Russia. Photo: AP.

Their conclusion: These initiatives weren't worth sacrificing for a deal on Syria, which was then lower on the foreign-policy priority list, say current and former officials who took part in the brainstorming exercise. Likewise, officials doubted such a gambit would work with a Russian leader whose motivations have confounded the U.S.

Now, Syria is the dominant foreign-policy challenge of Mr. Obama's second term. Russia is one of the biggest complicating factors. Moscow's unwavering opposition to striking Syria is the reason Mr. Obama couldn't get United Nations Security Council backing for military action against Mr. Assad, which in turn helped prompt the administration's risky gamble to seek political and legal cover from Congress.

The U.S. blasted Russia at the U.N. on Thursday, blaming it for preventing collective military action on Syria in response to Aug. 21 chemical strikes and endangering the international security system built to prevent such attacks. "Russia continues to hold the council hostage and shirk its responsibility," said U.N. Ambassador Samantha Power.

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The tension between the two powers has been aggravated by a series of miscalculations and misunderstandings. U.S. officials acknowledge they have struggled to understand Mr. Putin, a former KGB agent. Russia, for its part, bristles at what it sees as the U.S. tendency to use humanitarian abuses as cover to remove regimes it doesn't like, such as in Libya.

Mr. Assad's arsenal of advanced Russian-made weapons systems, including a recent shipment of upgraded Yakhont antiship missiles, has made Pentagon planning for the strikes more difficult, U.S. officials say. As a precaution, the U.S. Navy is keeping its destroyers far from the Syrian and Lebanese coast lines and out of range, the officials say. Lebanon is home to Syria's close ally, Hezbollah, which also has sophisticated antiship rockets.

As of Thursday, Russia had two warships, two support vessels and three amphibious troop and equipment movers off the Syrian coast, which U.S. officials say they believe are tracking American military movements in the area to share with the Syrian regime. U.S. officials say they believe Russian satellites and radar sites are also feeding information to the Syrian regime.

Mr. Putin said earlier this week that Russia would complete delivery of advanced S-300 air-defense systems to Syria if the U.S. strikes, which could shift the regional military balance.

Mr. Obama came to office promising a cooperative approach to Russia, a policy that yielded agreements in areas including arms control and sanctions against Iran during his first term. Since the return of Mr. Putin to the presidency last year, tensions have risen.

Aaron David Miller from the Woodrow Wilson Center discusses how a U.S. military strike might take shape in Syria, what kind of retaliation that could provoke from Damascus and what the crisis portends for America's allies.

Mr. Putin has argued in public that the Kremlin isn't per se committed to Mr. Assad, despite his status as a longtime arms client and host to a Russian naval facility—a stance he has repeated in private talks with Western officials, say people familiar with the talks. Instead, Russia opposes what it sees as a U.S. tendency to forcibly remove regimes it dislikes, which Russian officials trace to U.S.-led operations in Kosovo and Iraq, and most notably to NATO's operation in Libya in 2011.

Before the Libya vote, Russian officials say they were told by members of the U.S. delegation to the U.N., led by then-Ambassador Susan Rice, that the military mission in Libya wasn't designed to bring about "regime change." U.S. officials say they didn't mislead the Russians and say Moscow reviewed the language of the resolution in advance.

Mr. Putin, who was prime minister at the time, was skeptical of U.S. assurances but didn't insist on a veto, Russian officials said. He viewed the killing of Libyan leader Moammar Gadhafi by an armed Libyan militia as confirmation of U.S. deception. Late that year, he called Western actions an "outrageous violation" of the U.N. resolution.

Speaking to visiting foreign reporters and analysts, he added, "Syria is the next in line."

Russia's U.N. envoy Vitaly Churkin said the flap over Libya is a prime reason the Security Council couldn't act on Syria. "There is no trust. There is no confidence that if a resolution is adopted it is going to be used according to its terms and not twisted in a way which only some members of the Security Council are interested in," Mr. Churkin said in an interview.

Said White House National Security Council spokesperson Caitlin Hayden: "The president, Ambassador Rice and the text of U.N. Security Council Resolution 1973 itself were all very clear that the United States and the international community were squarely focused on the protection of civilians in Libya."

U.S. officials haven't found it easy to get a handle on Mr. Putin or his motivations. While anti-Americanism has been a key theme for Mr. Putin, the U.S. expected he would dial back the rhetoric once he had cemented control after the 2012 elections, a prediction that proved unfounded.

Mr. Obama's attempts to make a personal connection, such as complimenting Mr. Putin's judo skills during their last meeting, have fallen flat. Offhand comments, such as Mr. Obama's recent comparison of Mr. Putin to a "bored kid at the back of the classroom," rankled in Moscow.

The Central Intelligence Agency's classified personality profile of Mr. Putin, prepared by the agency for Mr. Obama and other policy makers, says he was bullied in his youth. It also describes Mr. Putin as insecure, according to American officials who have read it.

Russian officials dismiss such suggestions as cheap psychology.

Earlier this year, the Obama administration renewed its efforts with the Kremlin. The growing Syrian death toll, refugee crisis and influx of Hezbollah and al Qaeda fighters had made Syria a more pressing national security matter, U.S. officials say. During trips to Russia, Secretary of State John Kerry, among others, challenged Mr. Putin's comparison of the situation in Syria to the U.S. war in Iraq, noting he and Mr. Obama had voted against it. Mr. Putin wasn't convinced, according to people familiar with the conversations.

U.S. Defense Secretary Chuck Hagel has suggested the U.S. could buy goodwill from the Russians by assuring Mr. Putin the Russian naval base at Tartus will remain under Russian control. Mr. Putin has said Russia's priority is the principle of nonintervention, not the Tartus facility.

"The administration has a hard time understanding how Russia defines its own interests," said Thomas Graham, a Russia specialist who worked in the George W. Bush White House. "It is hard to buy them off when you are asking them to do things that in their view raise direct threats to Russian national interests."

A senior White House official said the U.S. wants to convince the Russian leadership it is in their interest to part ways with the Assad regime. "What we've said to the Russians is the United States isn't interested in removing Syria from Russian influence or acquiring Syria as a client state of the United States," the official said.

The idea of horse-trading with Russia didn't go anywhere in 2012. Today, the official said, it isn't an option either. Syria may be the "hot button issue of the day" but the White House still won't link it to what it sees as more-permanent national-security issues, the senior White House official said.

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